The present invention relates to a method of controlling air intake flow of an internal combustion engine.
The present invention may be used to particular advantage, though not exclusively, for controlling an internal combustion engine exhaust gas post-treatment system, and in particular for regenerating a nitric oxide adsorber for treating diesel engine exhaust gas, to which the following description refers purely by way of example.
As is known, diesel engine emissions comprise the following compounds, some of which are harmful to health and/or the environment:                carbon dioxide (CO2) and steam (H2O), both produced by complete combustion of hydrocarbons in the fuel;        unburnt hydrocarbons (HC) and carbon monoxide (CO), both produced by incomplete combustion of hydrocarbons in the fuel;        nitric oxides (NOx) produced by oxidation of nitrogen in the engine air intake; and        particulate, mainly produced by incomplete combustion of the injected fuel.        
Carbon monoxide and hydrocarbons are convertible to carbon dioxide and steam by the following oxidation process, which is active when the air-fuel mixture is lean, i.e. high in oxygen:CO+HC+O2→CO2+H2O
Nitric oxides, on the other hand, are convertible to carbon dioxide, nitrogen, and steam by the following reduction process, which is effective when the air-fuel mixture is rich:NOx+CO+HC->N2+CO2+H2O
Otto engines can exploit the above phenomena simultaneously.
To effectively eliminate all three of the above pollutants (HC, CO, NOx), the air-fuel mixture in the combustion chamber of Otto engines equipped with a trivalent catalyst must be stoichiometric, i.e. the amount of air fed into the combustion chamber must be the exact amount required to burn the fuel in the combustion chamber.
Trivalent catalysts, however, are unsuitable for use in diesel engines, operation of which calls for an air quantity in excess of stoichiometric proportions (lean operation), thus preventing reduction of nitric oxides in normal operating conditions, for the reasons stated above.
Nitric oxides produced by diesel engines, therefore, cannot be eliminated using a trivalent catalyst, and the effectiveness of a catalyst in this type of engine is limited solely to oxidizing carbon monoxide and hydrocarbons into carbon dioxide and steam.
To reduce nitric oxide emissions, a fraction of the exhaust gas is known to be recirculated into the combustion chamber (“EGR—Exhaust Gas Recirculation”). Exhaust gas comprises carbon dioxide, which, having a high thermal capacity capable of reducing the temperature in the chamber for a given heat produced by combustion, reduces heat exchange between the hot regions of the chamber, where nitric oxides are more easily formed, so that the overall effect is a reduction in the total amount of nitric oxides produced by combustion. The amount of exhaust gas fed back into the combustion chamber is normally regulated by a so-called EGR solenoid valve located along a recirculating line connecting the exhaust gas pipe to the intake pipe of the engine.
Used alone, however, exhaust gas recirculation fails to meet the requirements of more recent pollution regulations, particularly in terms of particulate and unburnt hydrocarbon production.
One recently adopted solution capable of effectively reducing nitric oxide emission is the use of a so-called nitric oxide adsorber (“NOx adsorber”), also known as a nitric oxide trap (LNT—“Lean NOx Trap”), which is fitted along the exhaust pipe, downstream from a conventional catalyst, and in which nitrogen monoxide (NO) is converted to nitrogen dioxide (NO2) by an oxidizing element, e.g. platinum (Pt), and then trapped in an adsorbent compound, e.g. barium oxide (BaO).
During the adsorption process, the barium oxide ultimately becomes unable to store nitrogen monoxide (NO), on account of saturation of the acceptor sites; and, when the nitric oxide adsorber reaches a saturation level at which nitric oxides are no longer eliminated effectively, the acceptor sites must be “cleared” periodically by so-called regeneration, i.e. desorption and simultaneous reduction of nitric oxides. At this stage, the barium oxide (BaO) is separated into nitrogen and carbon dioxide by a reducing element, e.g. rhodium (Rh), which is achieved by calibrating the diesel engine to produce a reducing environment (rich operation) in the exhaust gas for a few seconds.
Another factor which reduces the adsorption capacity of barium is the presence of sulphur in the fuel. Unfortunately, over 300° C., sulphur oxidizes to sulphur dioxide (SO2) which in turn may be converted by ambient humidity to sulphur trioxide (SO3). Both these compounds react with barium oxide in the same way as nitrogen dioxide, i.e. tend to become trapped in the barium acceptor sites in the form of barium sulphate (BaSO4), so that some of the acceptor sites are permanently occupied by barium sulphate, thus preventing retention of part of the nitrogen dioxide and so impairing efficiency of the adsorber. In fact, unlike regeneration of acceptor sites saturated with nitrogen dioxide, which takes place between 300 and 450° C., temperatures of around 600° C. are required to regenerate sulphate-saturated acceptor sites.
To prevent sulphates damaging the nitric oxide adsorber, the fuel must therefore contain no sulphur or, to limit the extent of damage, must contain at most 10 ppm.
Though slow, sulphate accumulation in the adsorber is therefore inevitable, on account of small quantities being derived anyway from the lubricating oil as well as the fuel, and must be removed periodically, every 1000-4000 km, by a specific regeneration strategy, known as desulphatization, combining reducing environment conditions and temperature levels of around 600° C.
Adsorption, desorption, and desulphatization are closely related to the composition of the air-fuel mixture during operation of the engine. That is, to adsorb nitric oxides and sulphur, the air-fuel mixture must be lean (oxidizing environment), whereas, to desorb nitric oxides or desulphatize sulphur oxides, the air-fuel mixture must be rich (reducing environment).
More specifically, the nitric oxide adsorption and reduction mechanism commences, in lean air-fuel mixture conditions, with oxidation of nitrogen monoxide (NO) to nitrogen dioxide (NO2) by the platinum (Pt) acting as a catalyst, according to the equation:NO+½ O2->NO2 
Subsequently, the nitrogen dioxide (NO2) reacts with the adsorbent element—barium oxide (BaO)—by which it is trapped (i.e. chemically sorbed) in the form of barium nitrate (Ba(NO3)2), according to the equation:BaO+NO2+½->Ba(NO3)2 
At the regeneration stage, the air-fuel mixture is enriched for a predetermined time period to increase carbon monoxide and unburnt hydrocarbon emissions and impart reducing properties to the exhaust gas.
The reducing atmosphere produces thermodynamic instability in the barium nitrate, which thus releases nitrogen monoxide (NO) and nitrogen dioxide (NO2), according to the equations:Ba(NO3)2->BaO+2NO+½ O2 Ba(NO3)2->BaO+2NO2+½ O2 
In rich air-fuel mixture conditions, and thanks to the presence of rhodium as a catalyst, nitrogen monoxide (NO) and nitrogen dioxide (NO2) are reduced by carbon monoxide (CO), hydrogen and hydrocarbons to nitrogen (N2) and carbon dioxide (CO2).
One possible reduction path is the equation:NO+CO->½ N2+CO2 
The air-fuel mixture is commonly defined quantitatively by the air/fuel (A/F) ratio or strength in the engine combustion chambers, which ratio indicates the amount of fresh air available for the combustion process.
Regeneration strategies currently comprise a fixed-duration (roughly 5-second) regeneration step, during which the air-fuel mixture is enriched—in particular, assumes an air/fuel (A/F) ratio value of between 12 and 14—and which is preceded by a fixed-duration (roughly 60-second) accumulation step, during which the air-fuel mixture is poor—in particular, assumes an air/fuel (A/F) ratio value of between 20 and 55.
One known method of modifying the air/fuel ratio in exhaust-gas-recirculation engines, to switch from the accumulation to the regeneration step, is to adjust the recirculated exhaust gas fraction, thus varying the amount of oxygen fed into the combustion chamber. European Patent Application EP-A-1 336 745 proposes a recirculated exhaust gas fraction control system, in which the EGR valve is closed-loop controlled so that engine air intake flow equals a reference airflow calculated on the basis of a desired reference air/fuel ratio in the engine combustion chamber.
The above closed-loop system of controlling air intake flow, however, is not very effective when regenerating the nitric oxide adsorber. That is, being of very short duration and depending greatly on the air-fuel ratio, regeneration of the nitric oxide adsorber calls for extremely precise, fast variation of the air/fuel ratio, which known control systems fail to achieve.
It is an object of the present invention to provide a method and device for controlling air intake flow of an internal combustion engine, in particular for regenerating a nitric oxide adsorber.
According to the present invention, there are provided a method and device for controlling air intake flow of an internal combustion engine, as claimed in claims 1 and 17 respectively.
According to the present invention, there are also provided a method and device for controlling an internal combustion engine exhaust gas post-treatment system, as claimed in claims 15 and 18 respectively.